Ettercap Allows for sniffing of machines in a switched network LAN (GPL, BSD/Linux/Solaris)

HUNT Allows for sniffing of machines in a switched network LAN as well as providing a very easy to use API to modify the intercepted frames before they are forwarded. Intercept and Modify. (GPL, Linux)

RRDtool is “a system to store and display time-series data (i.e. network bandwidth, machine-room temperature, server load average)”. (GPL, various UN*Xes) Many RRDtool-based applications are listed on the RRD World page.